1. Field of the Invention
The present invention relates generally to a spark ignition timing control system for an automotive internal combustion engine, which performs ignition timing control operation in both of anti-knock mode and a mode gradually or in stepwise advancing a spark advance toward a minimum advance for best torque (MBT) point. The control mode in which spark advance is advanced toward MBT point will be hereafter referred to as `MBT mode`. More specifically, the invention relates to spark ignition timing control system which accomplish both of suppressing of engine knocking beyond a predetermined level and an improved engine performance with improved transition characteristics during switching of operation mode from anti-knock mode to MBT mode.
2. Description of the Background Art
As is well known, performance of an automotive internal combustion engine becomes optimum when spark ignition is performed near MBT point, in view of fuel economy. As is also well known, the engine performance may be optimum when the engine knocking level is maintained at trace knock level. Therefore, in order to obtain optimum engine performance, spark advance is to be adjusted toward the MBT point. On the other hand, under certain engine driving condition, such as full load condition for acceleration, engine knocking level tends to be greater than the trace knocking level. As is well known, excessive engine knocking is quite harmful for the engine and tends to give a damage for the engine per se. Therefore, anti-knock spark ignition timing control becomes necessary for suppressing engine knocking. It is usual manner of anti-knock spark ignition timing control to retard spark advance gradually or in stepwise until the engine knocking level becomes lower than or equal to a predetermined level which substantially correspond to the allowable trace knocking level.
In view of the above, the spark ignition timing control is generally performed for seeking the optimum timing where the engine knocking level is maintained at the trace knocking level and the spark advance is advanced toward the MBT point as close as possible. Anti-knock mode control is always given higher priority than the MBT mode control since the engine knocking has to be suppresed regardless of the engine performance when substantial engine knocking occurs. In this viewpoint, it is usual manner of spark ignition timing control to advance the spark advance in MBT mode at smaller rate than rate of retarding the spark advance in anti-knock mode. Such spark ignition timing control system has been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-82074, For example. In the system disclosed in the above-identified Japanese Patent First Publication, the spark ignition timing is normally controlled in MBT mode so as to obtain optimum engine performance can be obtained with maximum engine output torque. For this purpose, in MBT mode control, the spark ignition timing is so adjusted as to perform spark ignition at a crank shaft angular position where the pressure in the engine combustion chamber is obtained will be hereafter referred to as `maximum pressure point .theta..sub.pmax `. The system also monitors engine knocking condition for detecting the engine knocking in excess of a given level in order to perform anti-knock mode control in response thereto. As set forth above, in the anti-knock mode control, the spark advance is retarded gradually or in stepwise until the engine knocking level can be lowered across the predetermined level. When the anti-knock control is successfully terminated, then MBT control is resumed to advance the spark advance gradually from the retarded angle in the anti-knock mode operation.
In such spark ignition timing control, recovery of the engine performance in the transition from Anti-knock mode control to MBT mode control is rather slow because the rate of advancing of the spark advance in the MBT mode control is much smaller than the rate of retarding of the spark advance in anti-knock mode control. This maintains the engine performance at low level for a long period of time after successfully suppressing the excessive level of engine knocking. This clearly degrade the engine drivability and fuel economy.
In addition, when engine knocking cannot be suppressed even by anti-knock mode control to retard spark advance for substantial magnitude, engine performance will becomes unacceptable low. In addition, excessively retarded spark advance may results in rising of the exhaust gas temperature to give damage for catalyst in the engine exhaust system.